Olefins are traditionally produced from petroleum feedstocks by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s), such as ethylene and/or propylene, from a variety of hydrocarbon feedstocks. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefin(s). There are numerous technologies available for producing oxygenates including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids or carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Other known syngas production processes include conventional steam reforming, autothermal reforming, or a combination thereof.
An important type of alternate feed for the production of light olefins is oxygenate, such as, for example, alcohols, particularly methanol and ethanol, dimethyl ether (DME), methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production. Methanol, the preferred alcohol for light olefin production, is typically synthesized from the catalytic reaction of hydrogen, carbon monoxide and/or carbon dioxide in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor. The preferred process for converting a feedstock containing methanol into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a catalyst composition.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
Catalytic processes utilizing fluidized bed technology for conversion of hydrocarbon or oxygenates involving gas-solids contacting are widely used in industry for productions of petroleum-based fuels, chemical feed stocks and other industrial materials. The gaseous reactants are contacted with solid catalyst particles to provide gaseous products. Such processes often use continuous catalytic reactor unit operations, requiring catalyst regeneration at high temperature. FCC, MTO and other processes usually employ oxidative regeneration to remove coke or other carbonaceous deposits from spent or equilibrium catalysts.
The hot vaporous product from the reactor, which contains light olefins, unconverted oxygenates, by-product oxygenates, and large amounts of water can be condensed in a suitable condensing means such as a quench tower, heat exchanger, flash drum, primary fractionator or other suitable device, to provide a mixed oxygenate/water stream. This stream can be provided to a fractionation tower designed and operated to separate the unconverted and by-product oxygenates as an overhead stream, and fairly pure water as a bottoms product. The fractionation tower overhead stream is typically mostly methanol (as unconverted feedstock). Given the large amount of water in the fractionation tower feed, the tower is typically quite large with a very high reboiler and condenser duty. Further, there are numerous locations in the overall MTO process which produce streams quite rich in methanol, but also containing certain heavy contaminants such as metal particles or ions. These streams tend to be liquid. It would be beneficial to recycle these methanol rich streams to the MTO reactor following removal of the heavy contaminants, and potentially following vaporization. There is also a desire to make the recycle of these streams as efficient as possible, to save utilities/energy in the overall process.
U.S. Pat. No. 6,121,504 to Kuechler et al. discloses a process for converting oxygenates to olefins with direct product quenching for heat recovery and to improve heat integration.
U.S. Pat. Nos. 6,403,854 and 6,459,009 to Miller et al. disclose a process for converting oxygenate to light olefins with improved heat recovery from reactor effluent streams and improved waste recovery which minimizes overall utility requirements. The reactor effluent is quenched with an aqueous stream in a two-stage process to facilitate the separation of hydrocarbon gases from any entrained catalyst fines, as well as to remove water and any heavy by-products such as C6+ hydrocarbons. A portion of the waste water stream withdrawn from the bottom of the quench tower is recycled to the quench tower at a point above where the reactor effluent is introduced to the quench tower. The references do not appear to teach the use of liquid streams that are substantially free of catalyst fines for treating reactor effluents.
U.S. application Ser. No. 09/998,512, filed Nov. 30, 2001, published Jul. 10, 2003 as United States Patent Application 20030130555, discloses a process for separating oxygenated hydrocarbon from olefin product of oxygenate to olefins reaction. The product is sent to a cooling unit such as a quench tower from which cooled olefin product is separated as an olefin vapor stream. The water containing bottoms stream can be recycled through a heat exchanger for cooling and/or removed from the cooling unit to a first separator, such as a distillation column to provide an oxygenated hydrocarbon product of reduced water content and remaining water as a bottoms product. The olefin vapor stream is compressed and directed to a second separator which provides an olefin vapor product and a liquid oxygenated hydrocarbon containing stream which can be combined with the water containing bottoms stream or directly added to the first separator to provide an oxygenated hydrocarbon product recovered from the first separator that is reduced in water content and can be used as fuel or co-feed for the oxygenate reaction process.
All of the above references are incorporated herein by reference in their entirety.